The simplest definition of enzymes is that enzymes are proteins which can act as catalysts for a chemical reaction. As with all proteins, the amino acid sequence and the three-dimensional structure of enzymes are both important to the proper biological functioning of enzymes. Some enzymes are not functional as enzymes in the normal form in which they are synthesized originally in living cells. Instead, these enzymes are expressed as an enzyme precursor known as a zymogen. A zymogen is acted on by some other enzymatic process in or out of the cell to convert the zymogen into the enzymatically active form of the protein. Typically enzymes which could cause damage to cellular biochemical processes, like proteases, are produced as zymogens, thus permitting their activity to be controlled more precisely. Zymogens are inactive as expressed, and thus can be stored or transported safely and then activated by proteolytic activity only when needed.
In living cells, biological polymers such as proteins and nucleic acids must continually be digested into their monomers which can then be recycled for use in making new proteins or nucleic acids. Proteases digest proteins and nucleases digest nucleic acids. Among the best studied of nucleases are the ribonucleases which are adapted to digesting RNA molecules. In particular, the sequence, the three-dimensional structure, and many aspects of the functioning of ribonuclease A (RNase A) have been characterized, particularly with regard to bovine pancreatic ribonuclease A. RNase A exists in cells in balance with a ribonuclease inhibitor (RI), which binds to RNase A to inhibit its activity. It has already been found that adding a moiety to RNase A that acts to provide a steric barrier to binding between RI and RNase A causes the RNase to be uncontrolled by the RI and therefore cytotoxic, presumably by digesting needed RNA species in the cell.
It is a characteristic of proteases that they have specific amino acid sequences in target proteins that they recognize to digest a specific peptide bond. Many types of cells or organisms have specific proteases that recognize specific amino acid sequences for their target cleavage site. It is thus possible to design proteins which will be enzymatically cleaved only by proteases present in a select group, type or species of target cells.
Other objects, features and advantages of the invention will become apparent upon consideration of the following detailed description.